This invention is concerned with a method of forming a workpiece made up of two portions of material secured together to form pockets therebetween, and also with a forming assembly for use in carrying out such a method.
There is described in our co-pending U.S. patent application Ser. No. 07/872902 a method or process of forming a workpiece made up of two portions of material secured together to form pockets therebetween, said method comprising the steps of inserting formers into said pockets, each former having a cross-sectional shape to be imparted to the pocket in which it is accommodated, and thereafter placing the workpiece, with the formers thus accommodated in the pockets thereof, between two presser members having corrugated presser surfaces corresponding to the cross-sectional shape of the formers such that adjacent pockets of the workpieces each together with its former are accommodated in adjacent cooperating corrugations of each presser member, and then applying forming pressure to the presser members and thus to the workpiece retained therebetween.
The method is particularly suitable, but not exclusively so, for the manufacture of self-supporting filter units having a plurality of passages defined by air-permeable wall portions through which air to be filtered can pass and be drawn off along said passages, but which prevent the passage therethrough of dust and/or other debris carried by such air. To this end, in the forming process the presser members are heated to a sufficiently high temperature to cause the material of which the workpiece is made up to be rendered rigid, while retaining its air-permeability.
In using such a method as described in the aforementioned patent specification the presser members each comprise a continuous corrugated presser surface into which, therefore, the workpiece, with the formers accommodated in the pockets thereof, has first to be arranged. In practice, the operator places the workpiece on top of a first presser member and ensures that each corrugation accommodates its own pocket with accommodated former, whereafter the other presser member is brought into surface contact from above and the workpiece is thus held between the presser members. In most cases, however, there is more "bulk" in the workpiece before forming than after, due to the application of heat and pressure in the forming operation. Thus, the loading of the unformed workpiece onto the lower presser member and maintaining it in position thereon while the other presser member is brought into surface contact therewith is time-consuming and tedious and may lead to unreliable location.